2-Fluorohistidine has been found a powerful growth inhibitor for E. coli. By mechanisms still unknown, the compound permits multinucleated cells to divide, but prevents duplication of genetic material. 5- Fluoroimidazole-4-carboxamide riboside has been found a highly effective antiviral agent and inhibitor of both DNA and RNA synthesis. 2- Fluorohistamine is an effective agonist for the histamine Hl receptor, but has little or no effect on the H2 receptor. 2-Fluorohistidine analogs of the hormone-releasing factors, TRF and LRF, show 20-30% of the activity of the natural peptides, despite a 5-unit decrease in pK. Methods have been developed for tritium labelling of 4-fluorohistidine. The incorporation of 2-fluorohistidine into animal protein is blocked by actinomycin and cycloheximide, or by administration of a large pool of histidine.